When airways are obstructed there is no further ventilation
to the lungs and beyond. In the early stages blood flow continues and
gradually the oxygen and Nitrogen get absorbed, resulting in atelectasis.

Relaxation atelectasis

The lung is held close to chest wall because of the negative
pressure in the pleural space. Once the negative pressure is lost the lung
tends to recoil due to elastic properties and becomes atelectatic. This occurs
in patients with pneumothorax and pleural effusion. In this instance, the loss
of negative pressure in the pleura permits lung to relax, due to elastic
recoil. There is common misconception that atelectasis is due to compression.

Adhesive Atelectasis

Surfactant reduces surface tension and keeps alveoli open. In conditions where there is loss of surfactant the alveoli collapse and become atelectatic.
In ARDS this occurs diffusely to both lungs. In pulmonary embolism due to loss of blood flow and lack of
CO2, the integrity of surfactant gets impaired.

Clinical history and physical findings should help in determining the most probable etiology for a given
atelectasis.

Special radiological signs

Bowing Sign

In LUL atelectasis or following resection, the oblique fissure bows forwards in the lateral view. Bowing sign refers to this feature.

Double Lesion Sign

If you encounter atelectasis of RUL and RLL sparing RML, it is difficult to comprehend a single endobronchial lesion to account for both lesions. There has to be two independent endobronchial lesions, hence it was proposed that it is unlikely to be due to primary bronchogenic cancer. The concept is based on knowledge of Anatomy.

Caution: Do not rule out lung cancer when you see this phenomenon, you just have to find an alternate explanation for the second lesion.

S curve of Golden

The transverse fissure is "S" shaped. The proximal portion of the fissure is convex because the tumor mass prevents the fissure from moving towards hilum. Whenever you see this shape to a fissure consider a mass causing
atelectasis.

Open bronchus sign

When air bronchogram is visible in an atelectatic lung, it
implies that there is no airway obstruction. It is more a trapped lung with
patent airways.

Inverted comma sign

Do not mistake a Azygous lobe to movement of transverse
fissure. Azygous lobe gives the appearance of an inverted comma.